The present invention relates in general to remote keyless entry systems for motor vehicles, and, more specifically, to reducing quiescent current draw in the vehicle-mounted receiver while providing for an increased transmission range between a handheld portable transmitter fob and the vehicle-mounted receiver.
Remote keyless entry (RKE) systems for use with motor vehicles are well known in the art. Such systems typically include at least one remote control device, which typically takes the form of a key fob having a radio frequency (RF) transmitter and several push buttons for manually initiating a desired remote entry function. A receiver and controller are typically provided in the vehicle for receiving the RF signals and controlling one or more vehicle devices in order to effect the desired vehicle function. In-vehicle devices which have been remotely controlled in such a fashion include door lock mechanisms, a vehicle trunk, interior and/or exterior vehicle lights, and a vehicle horn or other alarm.
A key fob must be small in size in order to facilitate carrying in a user's pocket or purse. Thus, miniaturized circuits and a small battery size are employed. Energy efficient microelectronic circuits and methods of operation are necessary in order to minimize battery consumption and maximize battery life. The key fob must also accommodate a transmit/receive antenna that is preferably hidden within the key fob because of esthetic and durability concerns. Therefore, the antenna gain that can normally be achieved is fairly low. The low antenna gain has constrained the operating range over which broadcasts between the portable fob and the vehicle receiver can be reliably received.
The vehicle-mounted receiver typically operates only when the vehicle engine is not running. The receiver draws its power from the vehicle battery which is not then being recharged by the vehicle alternator. Therefore, it is desired that the current draw of the receiver be very low so that the battery charge is not depleted too quickly. A typical specification for a vehicle-mounted receiver may be that average quiescent current (i.e., as it idles in a reduced-power mode with only circuits necessary to detect the presence of an incoming signal being operable) is less than about 2 mA. Even with efficient circuit design techniques, a typical receiver may draw about 10 mA while monitoring for an incoming signal. To reduce average current to an acceptable level, the receiver is operated intermittently. As long as the receiver is turned on once during the length of time of an incoming signal then the receiver can detect that a signal is present and can wake-up completely for processing a message. The receiver can be put in a sleep mode drawing only a few hundred microamps for most of the time. To facilitate the intermittent operation of the receiver, the transmitter may retransmit the message signal multiple time in quick succession. Alternatively, a message preamble normally included at the beginning of an incoming signal may be provided with a sufficient duration to ensure that it awakens the receiver. Longer transmissions, however, reduce the lifetime of the battery contained in the portable transmitter.
Another important characteristic of an RKE system is the distance within which the user must come to the vehicle in order to transmit a signal that is receivable at the vehicle. One method to increase effective range would be to increase the transmitter power. However, government regulations aimed at reducing the likelihood of interference with other transmissions are in place which limit the allowed transmitter power. As disclosed in copending U.S. application Ser. No. 10/960,657, filed Oct. 7, 2004, entitled “Remote Keyless Entry System With Two-Way Long Range Communication,” the peak transmission power (and thereby the range) can be maximized by using time-spaced message packets so that a higher peak power is averaged over a longer message time so that the average transmission power stays within the limit of the government regulations. However, when the transmitter is turned on for shorter periods of time, the length of time that the receiver can be in sleep mode is correspondingly shorter. The shorter sleep period results in an undesirable increase in the quiescent current of the receiver.